Sequential PROPHET-600 - 7-8 LFO-MOD; 7-9 POLY-MOD

To obtain a better understanding of the DAC function, it will be useful to actually
observe Vdac on an oscilloscope, while exercising various keyboard and control
operations. For example, the individual FIL and AMP CVs will be seen to follow
envelope movements. The effects of various modulation paths can be easily
demonstrated.
The two sets of seven voltages at the end of the strobe are the wheel and control
successive-approximation voltages used to detect changes in position. This technique
will be discussed with the ADC.
U414 drives four CV DMUX ICs, and the ADC comparator (see below). Referring for a
moment to sheet D, U329 (-CS05) and Y» U330 latch seven CV DMUX ADDRESS bits.
This data selects the destination S/H for each Vdac value. One of eight S/H on each
4051 is selected with lines ABC. The 10-13 (Inhibit) lines go low to enable the active
DMUX chip. (Vee of each 4051 is operated from -5V because this lowers the operating
impedance of the analog switches in the 4051. Vdac is always positive.)
There are two main types of CVs: polyphonic and monophonic. Polyphonic CVs are
those which vary with the keyboard. In this class are the six separate OSC A, OSC B,
filter, and amplifier CVs which are demultiplexed by U415, U416, and U417. Of the
eight CVs demultiplexed by U41S, six are monophonic, because they each go to all six
voices. Similar to the switch bits, these voltages are basic program parameters.
The basic S/H circuit for each of these consists of a low-leakage capacitor and a
BIFET opamp, which has an extremely high input impedance. The 4051 momentarily
connects Vdac to the S/H. The capacitor charges to the value of Vdac. It retains this
value while the demultiplexer strobes all the other S/Hs.
Figure 2-7 illustrates the output waveform of a typical S/H. Pure dc is desired. The
output from a typical S/H should basically not "droop” during the 5 ms interrupt cycle.
When troubleshooting the synthesizer for malfunctions affecting all voices, it is a
often a good idea to start by checking appropriate mono S/Hs.
BAD BAD BAD
Figure 2-7
SAMPLE/HOLD WAVEFORMS
TM600A 7/83
2-13